This invention relates generally to the conversion of methanol so as to produce gasoline and, more particularly, to the treatment of all or a portion of the effluent obtained from said conversion process in order to decrease durene.
The conversion of methanol to gasoline is an important area of technology which has the potential of becoming even more important as the supply of crude oil is diminished and/or increased in price. Particularly advantageous catalysts which are utilized in the conversion of methanol to gasoline are a special class of crystalline aluminosilicate zeolite catalysts of which HZSM-5 is the most preferred member. There are many patents and publications which describe the conversion of methanol to gasoline over said special zeolites, including U.S. Pat. Nos. 3,931,349; 3,969,426; 3,899,544; 3,894,104; 3,904,916; 3,894,102; the disclosures of which are incorporated by reference.
One particular problem residing in the conversion of methanol to gasoline over ZSM-5 type zeolites is that durene is produced in amounts higher than that expected from C.sub.10 aromatic equilibrium distributions. Once an aromatic ring is formed in the presence of unreacted methanol, alkylation to tetramethylbenzenes occurs rapidly, but the smaller higher melting durene molecule (1,2,4,5-tetramethylbenzene, melting point 175.degree. F.) diffuses out of the ZSM-5 pore much more rapidly than isodurene (1,2,3,5-tetramethylbenzene) or prehnitene (1,2,3,4-tetramethylbenzene). There have been various proposals advanced in order to control or minimize the amount of durene which is produced in the catalytic conversion of methanol to gasoline.
The proposals for durene reduction have generally fallen into two broad categories. One approach to the problem has been to vary the reaction conditions regarding the conversion of methanol such that durene is not formed at all or formed in small amounts. Approaches of this type are represented by U.S. Pat. No. 4,025,576 wherein it is stated that durene formation is less in the conversion of methanol if methanol is first converted to olefins and the olefins thereafter convereted to gasoline.
The second approach with regard to durene control was not to attempt to control the amount of durene which was formed in the methanol to gasoline reaction but to process the durene so formed in order to diminish it. It is in this latter area that the novel process of this invention is operative and it represents an improvement over the heretofore practiced teachings of the prior art.
In this connection, U.S. Pat. No. 3,969,426 is concerned with a process for the diminishing of durene produced in a methanol to gasoline process by reacting a durene-containing stream with low boiling aromatics such as benzene and toluene in order to transalkylate and thereby diminish the durene content.
The novel process of this invention represents an improvement over the teachings of the above-mentioned U.S. patent in that no additional aromatics are required. The instant invention is not concerned with the reaction of a durene containing fraction with aromatics, but rather, is concerned with an isomerization process wherein the durene portion is isomerized to more valuable products. It is interesting to note that in the methanol to gasoline process involving conversion of methanol over a catalyst such as ZSM-5 such as disclosed in heretofore mentioned U.S. Pat. No. 3,969,426; that very little benzene is formed. Therefore, in order to carry out the process of U.S. Pat. No. 3,969,426 with regard to durene reduction, a source of benzene must be available in order to react with the durene containing fraction. The novel process of this invention completely obviates the problem of benzene supply and, in fact, it is carried out in the substantial absence of benzene, i.e. no greater than 5 weight percent and more preferably no greater than 1 weight percent.